[Haskell-cafe] [iteratee] how to do nothing .. properly

[Sergey Mironov]

Ok. I've checked iteratee-0.8.3.0 and 0.8.4.0. Results are same.

Sergey

2011/6/2 John Lato <jwlato at gmail.com>:
> Hi Sergey,
> I can't explain this; maybe it's a bug in enumWith?  I'll look into it.
> Thanks,
> John
>
>>
>> Message: 20
>>
>> Date: Thu, 2 Jun 2011 02:46:32 +0400
>> From: Sergey Mironov <ierton at gmail.com>
>> Subject: [Haskell-cafe] [iteratee] how to do nothing .. properly
>> To: haskell-cafe at haskell.org
>> Message-ID: <BANLkTimMFRWgH9Nopt-eua+L7jQcGq+u=g at mail.gmail.com>
>> Content-Type: text/plain; charset=ISO-8859-1
>>
>> Hi. Would anybody explain a situation with iter6 and iter7 below?
>> Strange thing - first one consumes no intput, while second consumes it
>> all, while all the difference is peek  which should do no processing
>> (just copy next item in stream and return to user).
>> What I am trying to do - is to write an iteratee consuing no input,
>> but returning a constant I give to it. I thought (return a) should do
>> it, but it seems I was wrong as return actually consumes all unparsed
>> stream. iter6 experience tells me that (peek>>return a) is what I
>> need, but it's completely confusing and not what I expected.
>>
>> Thanks,
>> Sergey
>>
>>  import Data.Iteratee as I
>>  import Data.Iteratee.IO
>>  import Control.Monad
>>  import Control.Exception
>>  import Data.ByteString
>>  import Data.Char
>>  import Data.String
>>
>>  -- countBytes :: (..., Num b) => Iteratee s m a -> Iteratee s m (a, b)
>>  countBytes i = enumWith i I.length
>>
>>  iter6 = do
>>     h <- countBytes $ (peek >> return 0)
>>     s <- I.stream2list
>>     return (h,s)
>>
>>  iter7 = do
>>     h <- countBytes $ (return 0)
>>     s <- I.stream2list
>>     return (h,s)
>>
>>  print6 = enumPure1Chunk [1..10] (iter6) >>= run >>= print
>>  print7 = enumPure1Chunk [1..10] (iter7) >>= run >>= print
>>
>>
>> Here is example ghci session
>>
>> *Main> print6
>> ((0,0),[1,2,3,4,5,6,7,8,9,10])
>> -- read 0 items, returns 0
>> *Main> print7
>> ((0,10),[])
>> -- read 10 items (???) returns 0
>> *Main>
>>
>>
>>
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>